The present invention relates to the manufacture of printed circuits and, more particularly, to a process for improving the surface insulation resistance of printed circuit boards.
In the manufacture of printed circuits, a fundamental starting material is an insulating substrate material, typically composed of an epoxy or polyimide resin and typically glass-reinforced, having a thin layer of metal (e.g., copper foil) adherently bonded to one or both of its top and bottom substantially planar surfaces. From this metal-clad printed circuit board starting material, a variety of different types of processes can be carried out to selectively define on the surfaces those areas which will constitute conductive circuitry of the printed circuit and those areas which will be non-conductive. The hallmark of all such processes is an eventual etching of metal, including the original thin layer of metal, in selected areas to there expose the underlying insulating substrate.
In a typical process, for example, innerlayer circuits for multilayer printed circuits are fabricated from a copper foil-clad insulating substrate by arranging an etch-resistant material on the copper surfaces (e.g., of the foil per se or of a further copper layer built up thereon) in the positive pattern of the desired circuitry. The copper not covered by the etch-resist is then etched away down to the substrate surface, and the resist then removed, to thus provide a surface pattern of circuitry lines separated by insulating substrate areas.
In another typical process, such as for fabricating double-sided printed circuits or for providing circuitry patterns on the outer-facing surfaces of a multi-layer printed circuit, a copper foil-clad insulating substrate constitutes the starting material of the double-sided circuit or of the outer-facing layers, as the case may be. Through-holes for conductive interconnection of circuitry layers otherwise separated by insulating substrate material (including innerlayer circuits in multi-layer circuits) are drilled through the board or multi-layer composite, and the surfaces are then metallized (such as by electroless copper) to provide copper metal on the through-hole surfaces and to provide additional copper over the copper foil. An organic plating resist (e.g., from the application, imaging and development of a photoresist) is then applied to board surfaces to provide a plating resist pattern in the negative of the desired circuitry pattern, and additional copper is selectively built up on the non-resist areas via electroplating. Thereafter, an etch-resistant material (e.g., tin-lead or another organic resist) is selectively deposited onto the exposed copper areas not covered by the plating resist, and thereafter the plating resist is removed. The board is then treated with a copper etchant to etch away the copper areas which were previously covered by the plating resist, thereby arriving at a selective pattern of conductive circuitry and insulating substrate areas on the board surfaces. Typical processing steps thereafter may include stripping of the tin-lead or organic etch-resist followed by selective application of a solder mask. If tin-lead was employed as the etch-resist, another option is reflow and fusing of the tin-lead before application of solder mask.
Critical to the functionality of printed circuits is the electrical integrity of the selective conductive paths and areas, as provided by the selective areas of insulating material which separates them on the board surface. To this end, the insulating material used in producing the copper foil clad substrates as the starting material in printed circuit manufacture is chosen to have a high electrical resistance. The surface of insulating material exposed after selective etching during the printed circuit manufacturing process generally exhibits somewhat less resistivity than the original insulating material itself, sometimes as a consequence of incomplete etching away of copper, but more commonly as a consequence of the presence on the surface of metal species from compounds (e.g., zinc and/or chrome compounds) employed by board manufacturers in the process of adhering the copper foil to the insulating substrate, which metal species are apparently so intimately associated with the board surface as to resist complete removal in the copper etching process. The decreased resistivity of the insulating material surface brought about by the presence of these metal species can be tolerated in certain printed circuits where relatively large insulating areas separate conductive areas. However, the trend today is toward much more complex and dense circuitry patterns, and as a consequence poor resistivity of the insulating surface areas, and particularly latent conductive paths thereon resulting from retained metal species, can readily lead to undesired cross-talk and shorting between closely-spaced conductive areas.
Commonly-assigned Letize U.S. Pat. No. 4,978,422 addresses the foregoing problems and describes a process whereby the insulating areas of a printed circuit board which are exposed after a selective copper etching are thereafter contacted with an aqueous alkaline permanganate solution at conditions effective to remove from the insulating areas a sufficient quantity of metal species so as to improve the electrical resistance afforded by the insulating areas in the printed circuit, followed then by neutralization of any residual manganese species. According to the Letize '422 Patent, the contact with alkaline permanganate effects the removal of a thin surface layer of the insulating substrate material, and with it the metal species embedded in or otherwise associated with the surface layer.
Although effective for its stated purpose, the permanganate process of Letize is not without some disadvantages in commercial practice. One difficulty is economic, in that the necessity for two processing steps (treatment with alkaline permanganate, and neutralization of residual manganese species) adds material expense and extends processing time. Another potential difficulty is the risk posed by too substantial or aggressive an attack of the insulating substrate, and removal of more than desired portions thereof, by the alkaline permanganate solution if not carefully controlled.